Conventional developers used for development/visualization of an electrostatic latent image formed on an image bearing member in laser printers, copying machines, and like image output devices which embodies electrophotography technology are either a two component developer consisting of toner and carrier or a unary developer consisting only of toner. Magnetic brush development using such a two component developer produces superior image quality to other development methods and are also applicable in color imaging and relatively inexpensive. For these reasons, the development method has been in popular use.
An image forming apparatus embodying the conventional magnetic brush development includes a developer bearing member equipped with a cylindrical metal sleeve and a magnetic roller located inside the sleeve. The roller includes a permanent magnet (magnetic field generation means) in which an N pole and an S pole are provided alternately. By attracting the two component developer to the surface of the metal sleeve of the developer bearing member and rotating only the magnetic roller while fixing the metal sleeve, the two component developer can be transported to a developing area located opposite an image bearing member carrying an electrostatic latent image formed thereon. A development electric field acting between the developer bearing member and the image bearing member causes only charged toner to electrostatically adhere to the image bearing member, thereby forming a visible image.
A shortcoming of the image forming apparatus embodying the conventional magnetic brush development is so-called beads carry over. The term refers to a phenomenon in which the development electric field causes not only toner, but also carrier, to adhere to the image bearing member. The carrier adhesion results in a serious problem of defective images. Specifically, the carrier adhesion is a cause for white spots in image regions and smears in non-image regions. The latter defect is caused by the toner transported on the “adhering carrier” from the developer bearing member. The problem is becoming increasingly serious in recent years because the diameter of toner particles is progressively reduced in response to demand for high image quality and so is the diameter of the carrier particles in line with the toner diameter reduction. The magnetic binding force exerted by the carrier on the developer bearing member weakens with the decreasing carrier particle diameter. The need for a solution to beads carry over problems is increasing.
Beads carry over is governed by various control factors (e.g., magnetic properties of the carrier, the magnetic flux density from the magnetic roller, the diameter of carrier particles, and the electric resistance of the developer), and for the purpose of relieving it, allows for a relatively high degree of freedom in design. Meanwhile, a carrier particle is a semiconducting particle (core) having its surface covered with an insulate resin layer. The resin layer peels off due to external stress over time, and resistance decreases. As a result, carrier may be charged due to an external electric field and a so-called induced charging may occur. That makes carrier adhering more likely to happen. As discussed above, beads carry over is likely to increase due to changes of the resin coating layer of the carrier over time. A solution is needed not only to initial settings, but also to changes over time.
The induced charging will be described below in reference to FIGS. 16(a) and 16(b). As illustrated in FIG. 16(a), a two component developer 11 consisting of a carrier (magnetic carrier) 160 and a toner 161 forms a magnetic brush on the surface of a metal sleeve of a developer bearing member 12 in a developing area and in contact with an image bearing member 21. The carrier 12 has a magnetic roller (not shown) inside thereof. The area in which the magnetic brush contacts the image bearing member 21 and the toner 161 is transported from the developer bearing member 12 to the electrostatic latent image formed on the image bearing member 21 is termed the development nip section.
In this circumstances, if the DC bias voltage applied to the metal sleeve is −400 V and the metal body of the image bearing member 21 is grounded as illustrated in FIG. 16(a), the electric charge of the surface of the image bearing member 21 is cancelled in image areas. The surface electrical potential is therefore 0 V in those sections. Negative induced electric charge is thus injected from the development sleeve side to the carrier 160 on or near the tip of the magnetic brush due to an electric field generated in the development nip section. Due to the induced charge, a Coulomb force acts on the carrier 160 on or near the tip of the magnetic brush in the direction toward the image bearing member 21. If the Coulomb force is greater than the magnetic binding force acting on the developer bearing member 12, the carrier is adhered (transferred) to the image bearing member 21 (beads carry over), producing white spot image.
Meanwhile, in non-image areas, negative charge on the surface of the image bearing member 21 is remaining as illustrated in FIG. 16(b). If the surface electrical potential is −600 V, positive induced charge is injected from the metal sleeve side to the carrier 160 on or near the tip of the magnetic brush due to an electric field generated in the development nip section. Due to this induced charge, similarly to the caser for the image area, a Coulomb force acts on the carrier 160 on or near the tip of the magnetic brush in the direction toward the image bearing member 21. If the Coulomb force is greater than the magnetic binding force acting on the developer bearing member 12, the carrier is adhered to the image bearing member 21 (beads carry over) and carries the toner 161 with it, producing smears.
The problems of temporally decreasing carrier resistance, as well as resultant increasing beads carry over and deteriorating image formation quality, all detailed above disrupt stable high output image quality over an extended period of time. Various attempts have been made so far to solve the problems.
An example is disclosed in patent document 1. The image forming apparatus detects beads carry over or measures adhering carrier on the basis of the current flow through developer to regulate bias voltages for the image bearing member and the development device according to the detection/measurement. Generally, carrier is likely to adhere when the developer has a low volume resistivity, and the carrier adheres in increasing amounts when a bias voltage is increased in development. These particular issues are addressed by the image forming apparatus which decreases the bias voltage to cancel out the increase of beads carry over caused by decreasing volume resistivity of the developer. Adhering carrier is thus reduced.
Patent document 2 disclosed an image forming apparatus restrains increase of beads carry over by varying the AC duty ratio of a development bias. Specifically, the image forming apparatus includes a power supply in which a DC power supply and an AC power supply are connected in series. Current supply from the power supply to the developer is measured. The volume resistivity of the developer is calculated from the current measurement. A toner mix ratio is predicted from the relationship between the calculated volume resistivity, a predetermined volume resistivity, and a predetermined toner mix ratio. If the predicted toner mix ratio is below a preset tolerable minimum value, the AC duty ratio of the development bias is lowered. The operation reduces the electrical potential difference between the carrier and the latent image region on the image bearing member. This makes it difficult to inject more electric charge from the development electric field to the carrier, thereby restraining carrier from adhering to the image bearing member.
Patent document 3 discloses an image forming apparatus which measures degradation of a developer on the basis of an electric current through a developer and if the degradation measurement exceeds a preset reference value, performs a process to exchange the developer or similarly extend the life of the developer. Owing to the extended life of the developer, the image forming apparatus can continue forming good images. The device also addresses other problems, including the following ones. Charging capability of the carrier deteriorates due to toner being spent on the carrier and for other reasons, which hampers toner from being sufficiently charged and causes low image density. Another problems that can be addressed by the device is scattering toner and a resultant dirty interior.
[Patent document 1] Japanese Unexamined Patent Publication No. 4-34573/1992 (Tokukaihei 4-34573; published Feb. 5, 1992)
[Patent document 2] Japanese Unexamined Patent Publication No. 2000-98730 (Tokukai 2000-98730; published Apr. 7, 2000)
[Patent document 3] Japanese Unexamined Patent Publication No. 4-80777/1992 (Tokukaihei 4-80777; published Mar. 13, 1992)
Beads carry over is known to intensify with an increasing electrical potential difference between the developer bearing member and the image bearing member as described in patent document 1. Therefore, the volume resistivity of the developer needs to be set so as to keep carrier adhering below a specified amount in the region of the image forming apparatus where a bias is applied.
Now, a correlation between the development bias voltage and the number of adhered carrier particles will be described for the two component developer. FIG. 17 is a graph representing experimental results of measurement of the number of carrier particles adhering per A4 sheet of paper when a development bias voltage is applied to two types of two component developers each containing a carrier of a different volume resistivity from the other.
In FIG. 17, the coated carrier has a core carrier with a resin coating and exhibits greater volume resistivity than the core carrier. Therefore, the developer containing the coated carrier has greater volume resistivity. The development bias is the value Vopc-Vdev, where Vopc is the surface electrical potential of the image bearing member and Vdev is the voltage applied to the development sleeve of the developer bearing member. Negatively charged toner is used in this exemplary experiment. For that reason, a positive bias region indicates an image region (solid black image region), and a negative bias region indicates a non-image region (background region).
The experimental results shown in FIG. 17 indicate beads carry over occurring both in the image region and the non-image region. The results confirm, as mentioned above, that the carrier is charged both positively and negatively and, again as mentioned above, that the number of adhered carrier particles increases with increasing development bias, for the following reasons. An increase in the development bias increases the current in the developer and the induced electric charge. The results further indicate that beads carrier over occurs less with the developer containing the coated carrier than with the developer containing the core carrier. It is appreciated from that fact that increasing the volume resistivity of the developer is an effective way of preventing beads carry over and also that the volume resistivity of the developer, in other words, the current flow to the developer, needs to be managed and a solution needs to be devised according to changes over time in order to restrain beads carry over which intensifies with decreasing volume resistivity of the developer due to degradation over time.
Considering these phenomena, although the technology of patent document 1 alleviates beads carry over by reducing the development bias voltage, it leads to other electrical potential problems. The electrical potential difference between the developer bearing member and the image area on the image bearing member however decreases. That reduces the toner adhering in the image area and hence the concentration of an output image.
The technology of patent document 2 alleviates beads carry over by sensing a drop in resistance of the developer and changing the AC duty ratio of the development bias voltage. This is not trouble free either. Changing development conditions can lead to variations in image density, non-uniform imaging, poor resolution, and degradation of image quality.
The technology of patent document 3 determines degradation of the developer by sensing a decrease in current in the developer. In other words, the technology determines degradation based on an increase in the resistance of the developer and does not sense a decrease in the resistance of the developer. Therefore, the technology is not capable of alleviating beads carry over caused by a decrease in the resistance of the developer.
The present technology, conceived in view of these problems, has an objective of providing an image forming apparatus, an image forming method, an image forming computer program, and a storage medium for the program which, so as to enable formation of high quality images, prevents the carrier from adhering to the image bearing member due to decrease in the volume resistivity of the carrier as a result of degradion of the developer caused by use over an extended period of time.